Cetrorelix is a decapeptide with a terminal acid amide group that is used in the form of its acetate salt. The synthesis and some pharmacological effects are described in European patent application 299 402 (U.S. Pat. No. 4,800,191).
It should be possible to administer the active substance subcutaneously in a dose of 0.1 to 20 mg. Aqueous solutions of the decapeptide are unstable, and, therefore, autoclaving in the container used to distribute it is not possible. During conventional sterilization, using the prescribed conditions, the decapeptide tends to decompose. To obtain an injectable solution it was therefore necessary to develop a lyophilizate.
The amount of active substance in the solution to be lyophilized is, however, so small that, in low active substance concentrations, only a loose fluff results on the glass wall of the ampoule after drying the solution free of auxiliary substances, and this fluff is carried out of the vial with the stream of water vapor generated by the sublimation process. It is therefore necessary to use a bulking agent that forms a stable cake. In high concentrations, this auxiliary substance can be dispensed with. The following auxiliary substances may be considered as bulking agents: hexitols, in particular mannitol, glucitol, sorbitol, such as D-sorbitol, dulcitol, allitol, altritol (for example D- and L-altritol), iditol (for example D- and L-iditol), their optically active forms (D- and L-forms) as well as the corresponding racemates. Mannitol is used in particular, such as D-mannitol, L-mannitol, DL-mannitol, sorbitol and/or dulcitol, and, of these, D-mannitol is preferred. The hexitol used may also be composed of a mixture of the hexitols named, for example a mixture of mannitol and sorbitol and/or dulcitol. Since dulcitol is less water soluble than, for example, mannitol, the dulcitol content in the aqueous solution should not exceed, for example, 3 percent by weight. Mannitol and sorbitol, on the other hand, can for example be mixed in any ratio.
Apart from hexitol it is also possible to add other, conventional pharmaceutical auxiliary substances, such as amino acids, such as alanine, glycine, lysine, phenylalanine, asparaginic acid, glutaminic acid, leucine, lactose, polyvinylpyrrolidone, glucose, fructose, albumin and equivalent bulking agents. Urea and sodium chloride may also be used as bulking agents. The total amount of such substances in the solution which is used for freeze-drying, is for example 0-16.9 parts by weight, for example 0.1-7 parts by weight, based on 1 part by weight of cetrorelix. In the finished lyophilizate the total amount of such auxiliary substances may be up to 16.9 parts by weight, based on one part by weight of hexitol. In detail, the amount of such auxiliary substances depends on the amount of hexitol present and to such an extent that the total amount of hexitol and such other auxiliary substances in the finished lyophilizate may not be more than a maximum of 17 parts by weight, based on 1 part by weight of cetrorelix. If only 0.1 part by weight of hexitol is present in the lyophilizate, it is thus possible to have up to 16.9 parts by weight of other auxiliary substances; if, for example, 8.5 parts by weight of hexitol are present, the amount of other auxiliary substances may for example be up to 8.5 parts by weight, based on 1 part by weight of cetrorelix.
It was, however, found, during development work on the lyophilizate, that the active substance behaves in a widely variable and unpredictable manner during processing. The first batches gave good results, but it soon transpired that difficulties occurred during sterile filtration and faulty batches resulted.
It is known from the literature, for example from Powell, M. F.; Pharmaceutical Research, 1258-1263 (8)1991; Dathe, M: Int. J. Peptide Protein Res. 344-349 (36) 1990; Szejtli, J.: Pharmaceutical Technology International 16-22, 1991 that oligopeptides, particularly those with terminal acid amide function, tend to form gels. During sterile filtration this is apparent from the speed of filtration, indeed the increased viscosity of such solutions can often already be detected organoleptically. A gelatinous layer remains on the sterile filter. It is then no longer possible to prepare a medication with an exactly and reproducibly defined active substance content.
Table 1 lists various results of the first 11 batches.
The active substance contents fluctuate between 100% and 36%.
TABLE 1Cetrorelix acetateBatchDosageActive substance content % 1100 μg100 2500 μg100 3500 μg 90 4500 μg 36 5500 μg100 6500 μg 85 7 1 mg 80 8 1 mg100 9 2 mg10010 2 mg 8011 2 mg100
To avoid this gel formation, the literature lists the following additives which may be tried out on an experimental basis:
organic solvents may be considered, for example acetonitrile, n-butanol, tertiary butanol, ethanol, isopropanol, octanol and benzyl alcohol. It is also possible to use salts and buffer solutions, such as acetate buffer, citrate buffer, sodium chloride, sodium phosphate, sodium EDTA, sodium bicarbonate, phosphate buffer, guanidine acetate, urea.
Polymers may also be used, such as gelatin, polyethylene glycol 600, hydroxyethyl starch, polyvinylpyrrolidone, polyvinyl alcohol. The use of amino acids, for example alanine, glycine, lysine, phenylalanine, asparaginic acid, glutaminic acid and leucine has also been described. Acids that were used were citric acid, caprylic acid, octanoic acid, hydrochloric acid, sulphuric acid and acetic acid. Physiologically acceptable surfactants that may be used are benzalkonium chloride, cetyl alcohol, bile acids, lecithins, polysorbates, Spans® and Pluronics®.
Carbohydrates and cyclodextrins such as glucose, lactose, mannitol, saccharose, alpha-, beta- and gamma cyclodextrins, hydroxypropyl-alpha- and beta-cyclodextrins, hydroxyethyl cyclodextrins and methyl cyclodextrins have already been used. These auxiliary substances were tested as filtration supporting agents to prevent gel formation.
No satisfactory solution of the problem could, however, be found. Only acidification with acetic acid showed partial success. Here, too, it was, however, always necessary to accept high filtration losses.